There has hitherto been known a hydraulic braking apparatus as disclosed in Japanese Unexamined Utility Model Publication No. Hei 4-133003.
According to this prior art, a first and a second main circuit 3 and 4 which are included to connect a hydraulic motor 1 to an operating valve 2 are provided with a pair of check valves 5 and 5 and a braking valve 6. When the said hydraulic motor 1 is to be stopped upon returning the said operating valve 2 to its neutral position N, the said braking valve 6 will be brought into its blocking position so that it may limit a flow of the high pressure fluid that is discharged by the said hydraulic motor 1 with its pumping action to apply a braking force thereon. The said apparatus further includes a bi-directionally operating safety valve 7 provided on a short circuit path 8 which bridge a pair of portions 3a and 4a which are located in the said first and second main circuits 3 and 4, respectively, between the said hydraulic motor 1 and the said braking valve 6.
The above mentioned safety valve 7 will be held in a closed state by a spring 14, and it will be thrusted in an opening direction under a pressure of a first, a second pressure receiving portion 15, 16. It has a construction in which the said spring 14 is retained by a piston 17 that is adapted to slide by a predetermined distance and the said piston 17 has a pressure receiving chamber 18 which is connected to a pipe conduit 9, thus being capable of being provided with a high pressure fluid. The said safety valve 7 will be held in a low pressure set state when the said pressure receiving chamber 18 is not provided with the said high pressure fluid, and will be turned to a high pressure set state when the said pressure receiving chamber is provided with the said high pressure fluid to push the said spring 14 with the said piston 17.
This being the case, it can be seen that the said safety valve 7 has the two possible pressure stages, the first with a low pressure that is determined by the spring 14 alone and the second with a high pressure that develops when the said spring 14 is thrusted by the said piston 17.
The above mentioned pipe conduit 9 is connected to an internal passageway 10 of the said braking valve 6. For this reason, when the said operating valve 2 is set by a lever 13 in its neutral position N to allow a fluid that is discharged from a hydraulic pump 12 to flow into a reservoir 11, the above mentioned pressure receiving chamber 18 will not be supplied with a high pressure fluid. Also, when the said operating valve 2 is set to take a first position A or a second position B, the fluid discharged from the said hydraulic pump 12 will be supplied into the said pressure receiving chamber 18.
With such a hydraulic braking apparatus, a said safety valve 7 in the state in which a said operating valve 2 is in its neutral position N will have a relief initiation pressure that is reduced, thus capable of lowering the pressure in a said main circuit at the high pressure side while a said hydraulic motor 1 is performing a pumping action. This will allow any shock at the time of stopping the said hydraulic motor 1 to be reduced. On the other hand, the said safety valve 7 in the state in which the said operating valve 2 is set to take a said first position A or a said second position B will have a relief initiation pressure that is increased, thus capable of increasing the drive output of the said hydraulic motor 1.
With a hydraulic braking apparatus as constructed as mentioned above, however, it has been noted that due to the fact that the relief initiation pressure, i.e., the set pressure, of the said safety valve 7 has to be altered between the two stages depending upon the fluid discharged from the said hydraulic pump 12, the said safety valve 7 needs to of variable type which is extremely complicated in structure and costly.
It should also be noted that if the said operating valve 2 that has been in the first position A to cause the said hydraulic motor 1 to be rotated in a given rotary direction is suddenly operated to take the second position B (this operation will hereinafter be referred to as "reversed lever operation"), it follows that one of the two ports of the said hydraulic motor 1 will be switched to have a low pressure from a high pressure whereas the other port will be switched to have a high pressure from a low pressure.
Then, the set pressure of the said safety valve 7 will suddenly reduced from a high pressure to a low pressure and will thereafter be suddenly elevated to a high pressure again.
In other words, since the said operating valve 2 is switched from the first position A via the neutral position N to the second position B, the set pressure of the said safety valve 7 will suddenly be switched to have an elevated pressure from a low pressure during the speed reduction period.
On the other hand, the said hydraulic motor 1 that is designed to drive bodies of inertia (i.e., bodies each having a high inertia), e.g., traveling wheels, will be rotated in the direction in which the pumping action is effected by the inertia to discharge the fluid when the said braking valve 6 is switched to the opposite side during the above mentioned reversed lever operation. Then, due to the presence of the set pressure of the said safety valve 7, a braking force will be applied to the said hydraulic motor 1.
In this case, only immediately after the said reversed lever operation the said hydraulic motor 1 will be slowly braked with the set pressure of the said safety valve having being a reduced pressure. Since, however, the set pressure of the safety valve 7 is suddenly elevated to a high pressure in a midway of the braking operation, a peak pressure will be generated on a port side of the said hydraulic motor 1, thus bringing about a cause whereby the life of the hydraulic motor 1 is reduced.
It is accordingly an object of the present invention to provide a hydraulic braking apparatus whereby a shock at the time when the hydraulic motor is stopped can be reduced and the drive output of the hydraulic motor can be increased, whilst a said peak pressure at a port side of the hydraulic motor may not be generated, thereby eliminating a possibility for the said hydraulic motor to be damaged.